Correction for line distortion



I June 1,1926. I Y 1,586,82?

R. c. MATHEs CORRECTION FOR LINE DISTORTION Filed August 24 1920 m V l i 24 mn--'vm 4 LW N, N

IHVQ/JOFII Robe/ 7 C. Maf/ves.

I Patented June 1, 1926.

UNITED STATES 1,586,822 PATENT OFFICE.

ROBERT C. MATHES, OF NEW YORK, ,N. Y., A SSIGNOR TO WESTERN ELECTRIC CO]!!- ZPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

CORRECTION FOR LINE DISTORTION.

Application filed August 24,1920. Serial No. 405,700.

The present invention relates transmission systems over which currents of a range of frequencies are transmitted, and more particularly to certain provisions for securing a more uniform transmission of the currents of the difierent frequencies.

The invention is adapted specifically for use in multiplex carrier current systems where a comparatively wide range of frequencies is transmitted, but it is also suited to telephone lines or to any system in which it is desired to secure substantially uniform transmission of both low and high ,frequency currents. It is well known that the attenuation of a line conductor varies with the frequency, the currents of high frequency suffering a greater loss by attenuation than the currents of lower frequency. A common instance of this efiect is the distortion which occurs on relatively long telephone lines. The higher harmonics of the voice may be suppressed to such an extent that the voice is unintelligible at the. receiver. The greater the range of frequencies used, the more pronounced is the distortion. In multiplex signaling lines,-employin.[g high frequency Waves as carriers, the range of frequencies may be many times as great as the range represented in a speech wave. It is the particular object of the invention to provide for equalizing for the variable line attenuation for any desired range of frequencies, but especially for the relatively large range of frequencies employed in high frequency multiplex systems. v

The exact nature and ,objects of the inventionwill be clear from the following detailed description in connection with the accompanying drawing of which Fig. 1 represents a schematic. circuit diagram of a portion of a repeater circuit embodying the invention, and Fig; 2 shows curves which will be referred to in the description.

The line sections LW and .LE terminate at the repeater station in the balancing net separate frequency currents.

currents are modulated or otherwise ,con-' trolled in accordance with silgnals to be transmitted. The lines are shown -as associated througha repeater RE for repeating all of the currents on the line LW into the line LE. It is understood that a duplicate repeater circuit might and ordinarily would be employed for repeating in theopposite direction, in which case it would be connected between the taps 24 and25, but it is deemed suflicient for the purpose of describing the invention to show merely the one repeater.

This repeater is shown as comprising two amplifying stages, the first consisting of-the two thermionic discharge devices 1 and 2 connected in balanced relation as to each other, and the second comprising the discharge devices 3 and 4: similarly connected as to each other. These pairsof discharge devices are connected in tandem. The particular object of employing balanced connections of the amplifier tubes in each stage is to secure a substantially linear characteristic, i. e. a constant amplifying factor for all impressed electromotive forces, so that currents of a large number of frequencies can be amplified at the same time without producing modulation between the This type of connection has been found in practice to give substantially pure repetition of a complex wave involving a number of frequencies. However, for some purposes one amplifier tube in each of the two stages may be suflicient, and in other cases one stage of amplification may be sufficient. The discharge devices are preferably of the well known type comprising an evacuated tube in which are a heated cathode, a plate and a grid. The grids of the tubes 1 and 2 are connected to the potentiometers 5 and 6, and by means of the sliding contacts, the voltages impressed on the tubes may be varied. These potentiometers are connected across secondary coils 7, the primary 8 ofwhich is bridged across the line LW.

Space current. for the amplifiers 1 and 2 is supplied from the source 9 through the choke coils 10 and 11, respectively. The

current wave components in the output of the amplifiers 1 and 2 pass through the large blocking condensers 12 and 13 and are impressed upon the grids of the respective second sta' 'e amplifiers 3 and 4. The output of these amplifiers" is through the pgimary coils 14, which serve to conduct th the space current from the source 9 for these amplifiers and the variation "current components. These latter components are transmitted to the secondary coils 15 come more highly attenuatedin transmission over the line section.LW than currents of the lower frequencies, and also that similar attenuation will be roduced by the line sec tion LE. It is desirable, therefore, that at some point on the line meansshall be introduced for correcting or equalizing for the variable line attenuation. This correcting means might be introduced at any convenient point on the line, but it is preferably associated with the repeating circuit and is so illustrated in the present instance.

This correcting or equalizing means will be termed herein an attenuation equalizer and it may consists broadly of any suitable devices or circuit elements so proportioned in their parts and so related to the line as to exert on the currents transmitted over the line, an effect opposite tothat produced by the unequal line attenuation for the different frequencies and to such an extent that the arrival currents are substantially undistorted notwithstanding the line attenuation.

According to the present invention the attenuation equalizer consists of a network including resistance, inductance and capacity and is connected in shunt relation to the line. This network is preferably placed between the two amplifier stages, if a twostage repeater is employed. .By including the discharge devices, such as 1 and 2, between the line and the complex network, refiection losses and impedance reactions'be tween the line LW and the equalizing'neb work are avoided. In the preferred form,

series with a circuit including a condenser 18and an inductance 19, the whole combination being connected across the line between the amplifiers 1 and 3. In the absence of tubes 2 and 4, this network would be suificient. Where a balanced amplifier is used,

as in the present instance, this network is duplicated on the opposite side by the resistance 20 and the antiresonant circuit comprising capacity 21 and inductance 222 For rents transmited through the circuit.

fixing the normal potential of the grids of the second sta e amplifiers 3 and 4, the battery 28 may he included between the filaments and the respective networks as shown.

It is well known that an antiresonant circuit such as 18, 19 offers practically an infinite impedance to transmission through the circuit of currents of the frequency to which it is resonant, and the impedance increases more or less gradually as the resonant frequency is approached. This is illustrated in Fi 2 in which the curve AB represents for a fimited frequency range the relation between the impedance Z, offered by a-circuit of this type, and the frequency F of 1c resonant frequency at which substantially infinite impedance would be offered by the circuit would lie at some distance to the right of the curve shown in this figure. It is seen that as this resonant frequency is approached, the impedance increases. The shape of this impedance curve may be controlled and thus the form of correction may be controlled to a large extent by proper choice of condenser 18, inductance 19, and resistance 17. The function of the resistance 17 is mainly to flatten the impedance curve and to make the variation of impedance less rapid as the frequency increases. The curve AB represents conditions when the resistance 17 is small, and the curve CD represents the case when the resistance 17 is large. By controlling the amount of resistance in series with the antiresonant circuit, therefore, the rate at which the currents of the lower frequencies are shunted across the repeater in excess of the currents of the higher frequencies may be controlled, and the amount of distortion produced by the repeater in the currents transmitted through it may be regulated. Since the impedance of the correcting network in shunt relation to the line is higher for the high frequencies, these will be transmitted through the repeater with greater efficiency than currents of the low frequencies, and the distortion which the repeater introduces is therefore seen to be opposite in sense to the distortion produced by the line attenuation. If the resonant point of the circuits 18, 19 or 21, 22 were chosen to lie within the range of frequencies to be repeated, the impedance of the network with increasing frequency would reach a maximum and then diminish producing an effect opposite to that desired. According to the invention, the circuit 18. 19, and the circuit-21, 22 are each made resonant to a frequency equal to or above any of the frequencies to be repeated, and the portion of the curves AB, CD, etc, are chosen which will give the desired kind of distortion in each case. By way of example it maybe stated that in a carrier telephone repeater in which carrier frequencies from resasaa 5000 to 25000 cycles were used, the 'resistances 17 and 20 each had a value of about 1500 ohms, the inductances 19'1and 22 were each of about 37 millihenrys and the shunt capacity of the coils was. found suflicient so that no physical condenser 18 'or 21 was necessary. The resonant frequency of this combination proved to be about'26000 cycles,

1 which was the frequency most efliciently transmitted. In another instance, where the maximum gain was to be at a frequency of 10,000 cycles, the resistances hadthewalue of 3500 ohms each, the inductance 150 millihenrys, and the condenser .001 microfarads. These values aregivencby way of "mitting thereover aseries of'carrier wavesof respectively different frequencies, the at tenuation of said line being greater for the higher frequency waves than .for the lower,

0 a unidirectionally transmitting amplifier associated with said line for amplifying all of said carrier waves, and a common means also associated with said'line for compen-- sating for the unequal attenuation of said waves of the difi'erentfrequencies, said amplifier being connected between said line and said mean a v 2. In combination, a line having increasing attenuation with increasing frequency -9 of the currents transmitted thereover, and an attenuation equalizer associatedswith the line comprisinga circuit in shunt of said line including an anti-resonant circuit, the effective shunt impedance of which, as to,

"4 said line, increases with the frequency of the transmitted currents and a resistance inseries with said anti-resonant circuit for modifying the .rate of increase of the i m-. pedance of the shunt circuit with increasing 9 frequency. g v

' 3. A transmission system including two 'line sections over which electrical waves are being transmitted, a circuit including unidirectionally conducting devices connect-v ed in. tandem and interconnecting said line sections, and a network included in saidcircuit between said devices for.c0ntrol1ing the attenuation characteristics of said 1 system, said network comprising circuit 'ele-- m'e'nts proportioned asa whole to give gradually varying attenuation over the frequency range of the transmitted currents.

4. A transmission system. including two line sections, a circuit including unidirectlonally conduct ng repeater elements con-' nected in tandem interconnecting said line sections, and an attenuation equalizer in- ;cludedin said repeater circuit between said repeater elements. 4

-5. In a signalingsystem, a line divided into two line sections terminating in a repeating station, a repeater for transmitting currents of different frequencies between said line sections comprising a two-stage amplifier and an attenuating network besaid repeater circuit, said network compristween said stages connectedin shunt across ing elements proportioned to give a gradually changing attenuation over' the frequency range of the currents transmitted over said hne, 'to correct for the unequal attenuation of'said currents by said line.

6. A repeater comprising a pair of thermionicv discharge devices connected inbal- 'anced relation with each other, a second pair of thermionic discharge devices c'onnected in balanced relation with each other, and in tandem as. to said,first pair, and a circuit bridged across the connection be; tween said pairs of devicesincludlng a cir cuit antiresonant to a frequencyoutside the range of. frequencies of the currents to be transmitted by said repeater and a resistance in series with said antiresonant circuit.

7 In a transmission system, a transmission line the attenuation of which varies withthe frequency, a repeating system associated' with said transmission line and comprising an impedance, said repeating system including two amplifying-- transmission paths,one for transmitting in one direction and the other for transmitting in.

the opposite direction, and an auxiliary circuit whose attenuation varies with the frequency 1n 'amanner complemental to that "of the line, said auxiliary "circuit being included in one of the transmission paths of the repeater system and being associated With the line only through said impedance.

8. A transmission system including two no a line sections over which electromagnetic I WfiVGS' are being transmitted, means for terminating one of said line sections in an impedance approximately equal to the impedance of the line section itself; a bridge across said line section including the inputv circuit ofa vacuum bulb amplifier, means associated with the output circuit of said amplifier for causing different frequency components of said waves to be transmitted with different efiiciencies, and a second a'm- P q associated wnh theeth f aid line sections. Y

my name this 10th day of August, ,A D, 1920. y

nearer O.- MATHES.

fplifier having its input circuit associated I with said last mentioned means and its out- 3 i In witness whereof, ll hereunto subscribe x 

